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WT versus Double angles

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I am reproducing a query posted on the steel-detail
list and my reply.
Any comments or additional thoughts / opinions are
welcome.
Regards
Vish
========

Wayne Byce wrote:

I usually try to keep a low profile here not interject
a lot, because
frankly, I don't like to show my ignorance. 

Question. What seems to be the biggest advantage for
using WT's for 
cross bracing as opposed to an angle with a comparable
cross section area?
==============================
Jay Gold wrote:

No advantage that I can see from the fabie side.

	1. Tees have to be split from Beams
	2. Straightened usually
	3. If single pieces they do not stack worth a s--t
	
Angles are nice we just put a program in the Angle
line load the Drive 
rack and watch them fall of the other end of the line.
The WT cost is more the my 2 cents worth.
==================================

 
Wayne,

I have never been shy of admitting ignorance.
My ignorance exceeds my knowledge by a factor of over
a million.

I have dispelled considerable ignorance by admitting
it and seeking enlightenment.

I started out on this list, over 5 years ago, being
ignorant about the most important thing in this
business and that was how much to charge for
detailing!!!

I made a public international spectacle of myself by
asking that prohibited question but the education I
received from all the answers I got compensated for
the public flogging I also received from all around.

That's enough philosophy for the day.

As regards your question on double angles versus WT
sections, I am inclined to agree with Jay Gold.
He has put it succinctly, as a hard nosed fabricator
who values his time.
As a normal nosed designer with plenty of time on his
hands this Sunday, let me ramble a bit on this
subject.

I generally preferred double angles.

Occasionally I used Tee sections but that was because
it offered me a way to use up extra W sections
procured or left over with the client from a previous
project and information and a bill of material was
furnished to us in advance.

In a double angle section, you have to use stitch
plates at intervals and that adds to the cost.
The spacing depends on whether the member is designed
for compression (needing closer spacing) or if it is
designed for tension.
This adds to the cost.

In a Tee section, you have to spend on a long cut of a
nicely manufactured W section. If the steel mill
could, it would wince. I can imagine it complaining 
"Why mutilate me ? "

>From design point of view, for the same area, a double
angle section is likely to give you better properties
(radius of gyration will be more, hence slenderness
ratio will be less and allowable compressive load will
be more).

(If any detailer wants to understand what exactly this
"radius of gyration" thingee  is, I can explain in
layman's language in a separate mail.)

A double angle section has another advantage as left
overs can be more readily used up elsewhere. Clips,
edge angles, seats, pour stops, ladders all use angle
sections. 

I  once specified a Tee section in a situation where
the section needed to be very compact due to
interference with a wall. The bracing plane was too
close to a wall for comfort. The toe of the equivalent
designed double angle in the vertical bracing
interfered with the face of the wall. The Tee cut from
a carefully selected W section just cleared the
surface.

The Tee is better as regards painting costs.  The
surface area is less. All the surfaces are accessible.
In a double angle the gap between the angles cannot be
accessed for painting later.

Theoretically, the most optimum section for a bracing
compression member is a  tube.
However tubes are costlier than rolled sections in
India. The range available is severely limited.

The tube thicknesses available in India were sometimes
not sufficient for taking corrosion into account.

Connecting it with bolts on the surface was not
practical. The bolts will need to go through the tube.

A grim and determined iron worker who routinely
tightens bolts all day is unlikely to remember that
tubes need to be handled gently. But Iron workers in
India reserve that for their love making in bed. In
the field they are at war. In all likelihood the  last
twist of his wrench, accompanied by a satisfied grunt,
will dent the tube surface and leave the burly iron
worker clucking his tongue in sympathy for this
"hollow  weakling" that couldn't withstand a simple
tightening effort from him.

The bolt lengths will also be excessive. Therefore,
for a good connection,  the tube needs to be cut short
at the ends, a longitudinal slot is to be made in the
tube  and a gusset needs to be inserted into a this
slot and welded. Two "half width" end plates are also
needed to seal the ends of the tube so that rain water
does not invade and take safe sanctuary inside and
cause corrosion havoc later. 

While the engineer, empowered with his sophisticated
analysis and design software,  will be delighted with
the "optimum weight design"  he would have achieved
using tubes, the poor fabricator will tear his hair in
despair. A simple straight and neat member all along
its length, gets needlessly complicated at the ends.


For this reason I avoided tubular bracing members and
chose  "starred" double angles, as the term goes in
India. (A pair of angles is used with both legs of
both angles outstanding so that the cross section
resembles a plus sign (+)

The radius of gyration is equal in both directions and
superior to any other configuration using double
angles. Theoretically this gives the maximum
compression load capacity.

But the pack plates need to be twice as long. They are
needed in both legs. More cutting of plates, more
stitch welding. More cursing from the fabricator, most
of whom would prefer projects in which they need only
procure correct lengths from mills, and drill the
correct number of holes in the right places and
dispatch them to the field for the erector to do his
job. They hate welding of any kind. Slotting and
coping of members, cutting and welding end plates,
stiffeners etc are positively anathema to them.

In industrial building design, (in which I was
involved for 26 years in the previous century), this
starred angle configuration would sometimes cause the
technologist to frown. He would be worried that the
extra projection of an inch or so would eat into their
sacred technological space inside the building. 

These technologists, who dictated terms, always
considered "bracings" an unnecessary nuisance to be
lived with and minimized and also strictly shown their
due places in the building and not allowed to hob-nob
with the more important technological structures. 

In the caste hierarchy, their equipment and machinery
were supreme. Piping and cables occupied a less
exalted position. Structural bracings( whether
horizontal or vertical)  were the dregs of
technological society, to be merely tolerated and
never encouraged.  Beams were better tolerated but
they always wondered why they needed to be so deep and
cause headroom problems for them. On many occasions
our bracings, located optimally and silently rendering
yeoman service during winds, earthquakes, crane surges
etc, would be shooed away with disgust, but that is
another story for another day.

The architects (if any were involved) would also find
starred angle configurations unwelcome as it might
clash with wall or panel positions. We are not allowed
to wonder why they need to be so thick as to interfere
with our bracings.

Finally, we come back to the question.
Which is better, WT or double angles?
You can't generalize. 
The engineer has to decide on  a case-to-case basis
and hope he is not overruled by the
technologist/architect.

My vote is for  a double angle unless it is ruled out
for a specific reason.

That ends my Sunday ramble.
Did you really read all this?
Thanks!

Regards
Vish


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